To reduce the size of micro-structured devices like semiconductor circuits (e.g. integrated, analogue, digital or memory circuits, thin-film magnetic heads) with the technique of optical lithography the optical resolution limit of optical microlithographic projection exposure systems must be further improved. Due to diffraction, the resolution limit in a first order approximation is inverse proportional to the numerical aperture of the projection lens of the microlithographic projection exposure system, with which structures are projected from a mask onto a substrate by a projection beam, to form the micro-structured devices there, e.g., by exposure of a light sensitive resist (which covers the substrate) with at least parts of the projection beam. For this reason, one focus is to increase the numerical aperture of the projection lens. Another focus is to reduce the used wavelength for the projection process, since the optical resolution limit is also proportional to this wavelength. For this reason the historical development of optical lithography systems was such that the wavelength of the used light in the projection process was reduced from visible light to ultraviolet light and now to Very Deep Ultra Violet light (VUV light, like 193 nm which is produced e.g. by an advanced ArF excimer laser). Now VUV lithography is broadly used in mass production of semiconductor circuits. Today, mass production of high integrated circuits is mostly done on microlithographic projection exposure systems with a projection light of the mentioned wavelength of 193 nm, whereas the numerical aperture NA of the projection system which projects the structures on a mask (or structured object) onto a substrate, is much more than 1.0, even more than 1.3. Such high numerical apertures only can be achieved by the use of immersion systems. The principles of such systems are already described e.g. in DD 221563 A1 or in US 2006092533 A1.
For an onward reduction of the size of the micro-structured devices, a further reduction of the wavelength of the projection light is necessary. Since in the very deep ultraviolet wavelength range almost all optical materials become opaque, there are no suitable materials for optical lenses for wavelength below about 157 nm. Using even shorter wavelengths for the projection light, the projection lenses can only work with reflective optical elements like mirrors or like diffractive optical elements. During the last years, enormous efforts were done to develop optical microlithographic projection exposure systems, which use for the projection process wavelengths less than 50 nm. Systems working with a projection wavelength between 10 nm and 14 nm are described e.g. in EP 1533832 A1 or in US 20040179192 A1. Depending on the light sources which are available for the projection light of such short wavelengths, the wavelengths for the projection light may be even 5 nm or less. At such short wavelengths of less than 50 nm or even much shorter, the projection lenses of the optical microlithographic projection systems comprise only reflective optical elements like mirrors and/or diffractive structures like reflective diffractive structures. Projection systems which are working at a wavelength of less than about 50 nm are known as EUV (Extreme Ultra Violet) lithographic projection exposure systems.
A simplified EUV lithographic projection exposure system 100 is schematically shown in FIG. 1. The system comprises an EUV light source 1, producing EUV light with a significant energy density in the extreme ultraviolet or EUV spectral region, especially in the wavelength range less than 50 nm, preferably in a range between 5 nm and 15 nm. Discharged-produced or laser-produced plasma light sources are used as EUV light sources, making use of e.g. xenon, tin or lithium plasma which generates the extreme ultraviolet light. Such sources irradiate unpolarized light under about 4π solid angle. Other sources generate a spatially more directed and a more polarized beam of extreme ultraviolet light like e.g. synchrotron radiation sources. Dependent on the EUV light source 1, especially if an EUV plasma light source is used, a collector mirror 2 may be used to collect the EUV light of the light source 1 to increase the energy density or irradiance of the EUV radiation and form an illumination beam 3. The illumination beam 3 illuminates via an illumination system 10 a structured object M. The structured object M is e.g. a reflective mask, comprising reflective and non-reflective or at least minor reflective regions to form at least one structure on it. Alternatively or additionally, the structured object comprises or consists of a plurality of mirrors which are arranged about side by side in at least one dimension to form a mirror arrangement like a mirror array. Advantageously the mirrors of the mirror array are adjustable around at least one axis to adjust the incidence angle of the illumination beam 3 which is irradiated on the respective mirror.
It shall be understood that the terms reflective, minor reflective and non-reflective relates to the reflectivity of EUV light of the illumination beam 3. Due to the very short wavelength of the EUV light, the reflective surfaces are usually coated if the angle of incidence for the EUV light is less than about 45°. The coatings preferably comprise a multilayer of predetermined layer materials with predetermined layer thicknesses. Such mirrors are usually used for incidence angles less or far less than 45° down to about 0°. For such mirrors a reflectivity of more than 60% can be achieved due to a constructive interference of the reflected EUV light which is partially reflected at the various material boundaries of the individual layers of the multilayer. A further advantage of such multilayer-coated reflective mirrors or surfaces is their property to work as a spectral filter, to make e.g. an illumination and/or projection beam of the EUV lithographic projection system more monochromatic. In an EUV lithographic projection exposure system coated mirrors are sometimes also designated as normal incidence mirrors.
For larger incidence angles than about 45°, especially for much larger incidence angles like angles of about 70° and even more, it is sufficient if the reflective surface comprises a metal or a metal layer like Ruthenium, or if the reflective surface consists of a metal or a metal layer, comprising e.g. Ruthenium. At such high incidence angles, the reflectivity can be increased up to 60% and more without the necessity of a multilayer as mentioned above. As a general rule the reflectivity increases with increasing angle of incidence, which is why these mirrors are also called grazing incidence mirrors. EUV lithographic projection exposure systems often use plasma light sources. In this case, the collector mirror 2 can be a grazing incidence mirror as described e.g. in U.S. Pat. Nos. 7,460,212, 7,244,954, 7,015,489, 6,964,485 or US 2004/0130809 A1.
The structured object M reflects parts of the illumination beam 3 into a light path which forms a projection beam 4. The structured object M structures the illumination beam 3 after being reflected on it, depending on the structure on the mask M. This projection beam 4 is carrying the information of the structure of the structured object and is irradiated into a projection lens 20 such that at least two diffraction orders of the structure or the structures of the structured object M pass the projection lens 20 and form a kind of an image of the structure or the structures of the structured object M on a substrate W. The substrate W, e.g. a wafer, comprising a semiconductor material like silicon, is arranged on a substrate stage WS which is also called wafer stage.
In addition to the information about the structure of the structured object M, the projection beam also comprises information about the illumination condition of how the structured object M is illuminated regarding angular, polarization and intensity (or radiation power per unit area) in an object point OP of the structured object M, and of how these parameters are distributed over the illuminated surface of the structured object M. The kind of illumination is expressed by the term “setting”. This means a predefined angular and/or polarization and/or intensity distribution with which an object point OP on the structured object M is illuminated, and how these distributions depend on the spatial position on the structured object M. The setting also influences the optical resolution of the projection process which is done by the projection lens 20. In general, the optical resolution can be increased if the setting is adapted to the shape of the structure on the structured object M. Advanced illumination techniques which use adapted settings for the illumination of a structured object are described e.g. in “Resolution Enhancement Techniques in Optical Lithography” by Wong, Alfred Kwok-Kit; ISBN 0-8194-3995-9”. The kind of illumination, the setting, can be adjusted with the illumination system 10 (see FIG. 1), which comprises a plurality of mirrors 12, 13, 14, 15, 16.
In FIG. 1, as an example, the projection lens 20 schematically shows four mirrors 21, 22, 23 and 24 as reflective optical elements to form a kind of an image of the structure of the structured object M on the wafer W. Such EUV projection lenses 20 typically comprise 4 to 8 mirrors. However, projection lenses with only two mirrors may also be used. These mirrors are made with highest precision regarding surface figure (or regarding their geometrical form) and surface roughness. Each deviation from the desired specification results in a degradation of the image quality on the substrate or wafer W. Usually the specification is such that a deviation from the surface figure (the required or specified dimensions of the shape of the surface) of less than one tenth of the used projection wavelength is required. Depending on the used wavelength the surface figures of the mirrors 21, 22, 23 and 24 must be made with a precision of even better than 1 nm, for some mirrors the precision requirements are even a factor of 5 to 20 higher, ending up at precision ranges of much smaller than one atom layer, which means better than 0.1 nm. To project structures from a mask to a substrate with the EUV lithographic projection technique in such a way that the image on the substrate comprises structures down to about 10 nm of lateral dimension or even structures with smaller lateral dimensions the optical aberration of the projection lens 20 must be smaller than 1 nm, even smaller than 0.1 nm or smaller than 50 pm (picometer) of RMS value. This means that the root-mean-square (RMS) value of the deviation of the real wavefront from the ideal wavefront is smaller than the mentioned values. This very high precision regarding the surface shape (surface figure or geometrical form) must be kept over a mirror dimension of more than 10 cm. Modern EUV projections lenses comprise mirrors of a diameter of 30 cm or even more with such a high requirement regarding the surface figure. This very high mechanical precision is necessary to form an image point IP on the substrate W from an illuminated object point OP on the structured object M by illuminating the object point OP with a well configured illumination beam according to a predetermined setting. Further, to project the illuminated object point OP with the projection lens 20 with at least parts of the projection beam 4 onto the substrate W, the projection beam 4 is generated by the illumination beam 3 and the diffracting properties of the structured object M. One necessary condition to form an image on the substrate W is that the diffracted wave fronts, which are coming from an object point OP, interfere in the image point IP on the substrate or wafer W. To get a good image quality the interfering wave fronts must have a relative phase shift of far less than one wavelength of the projection beam light. Due to the various illumination settings, of how the structured object M can be illuminated by the illumination beam 3, the light path of the light passing one object point OP on the structured object M can vary within the projection lens 20 in such a way that light bundles of the projection beam 4 are reflected by the mirrors 21, 22, 23, 24 of the projection lens 20 at different surface areas with different sizes. This variation depends on the illumination settings and the position of the mirrors 21, 22, 23, 24 within the projection lens 20. To make sure that the image quality is achieved under all illumination settings it is necessary that the above-mentioned surface figure is achieved with the mentioned high mechanical precision.
Apart from the high mechanical precision of the surface figure of the mirrors 21, 22, 23, 24 in the projection lens 20, also the position and orientation of these mirrors 21, 22, 23, 24 relative to each other, relative to the structured object M and relative to the substrate W must be in the same range of accuracy. This means that position and orientation of these objects (mirrors 21, 22, 23, 24, structured object M and substrate W) must be adjusted also in the nanometer range or even below. In addition, a metrology is necessary to allow the manufacturing of such precise mirror surfaces, the assembling of the projection lens of the EUV lithographic projection system, the integration of the assembled projection lens into the projection system, and to allow any in-situ monitoring and control of the system during the operation of the system.
To achieve the above mentioned mechanical precisions, one further problem is the absorption of the projection beam 4 by the mirrors 21, 22, 23, 24. This absorption which could be in a range of up to 30% heats the mirrors. Depending on the absorbed heat each mirror may be deformed due to thermal expansion of the mirror. One method to reduce such thermal effects during the projection step, when a certain amount of the projection beam 4 is absorbed, is to use a temperature control system to keep the very high mechanical precision data as mentioned above, especially for the surface figure of the mirrors. Another or an additional method is to use as a mirror material or as a support structure for the mirror a low thermal expansion material with such a small coefficient of thermal expansion (CTE) like 5 ppb/K (or less) to reduce deformations of the mirror, if the temperature changes e.g. due to partial absorption of the projection beam 4. This method and the selection of appropriate materials with the respective small CTE and the control of the mirror temperature is described e.g. in U.S. Pat. No. 7,295,284 B2.
EUV lithographic projection exposure systems like shown in FIG. 1 are usually operated under vacuum conditions. The projection lens 20 and/or the illumination system 10 are operated under reduced pressure or vacuum. Usually the pressure conditions in the illumination system and the projection lens are different. The reduced pressure or vacuum conditions significantly reduce the technical solutions for the above mentioned problems regarding the deformation of mirrors and their active or passive position control. Especially temperature control systems for controlling a temperature of components inside the EUV lithographic projection exposure system are quite often limited to certain technical solutions which are not essentially based on thermal convection principles.